Leaf nitrogen distribution to maximize the canopy photosynthesis in rice

The optimum distribution of leaf nitrogen (N) in the canopy of rice plants (Oryza sativa L.) for maximum daily canopy photosynthesis (DCP) and the optimization effects on DCP were estimated during the grain filling period. The low- and high-density canopies (28.3 and 47.5 plants m−2) and isolated plants were established at heading using plants in pots grown up at the low density until heading to make the same canopy architecture except plant density and the same leaf N distribution at the start of treatment among the two canopies and the isolated plants. The simulation was conducted under two conditions of the upper limit of leaf N. Under condition 1, upper limit of leaf N content was 1.80 g m−2. Under condition 2, upper limits were measured leaf N content in each leaf position at heading. The model indicates that if leaf N content in the upper leaves can be increased with reduction of N in the lower leaves, DCP will increase in any of the plant density, light conditions and under conditions 1 and 2. On a clear day, the estimated increase in DCP was 19–45 and 38–70% in the low- and high-density canopies under condition 1, respectively. Even under condition 2, which is more realistic than condition 1, the increase was up to 21 and 25% in the low- and high-density canopies. These estimates obtained by the present model that incorporates the shading effects of panicles and stems on DCP were higher than the previous reports which did not consider the effects of shading by panicles and stems. In the observed leaf N distribution, the higher the plant density was, the steeper the gradient of the leaf N remained. The gradient in the high-density canopy was closer to that of the predicted optimum leaf N distribution, and likely to contribute to maintaining higher DCP in the canopies. Compared with the hypothetical case in which gradient of leaf N distribution would be more gentle as observed in the isolated plants, the maintained steeper gradient of observed leaf N content in the canopies was estimated to increase DCP by 13 and 5% in the high- and low-density canopy, respectively.